Electroluminescent displays can have selectively illuminable regions for displaying information. Such displays have the advantage over competing technologies that they can be large, flexible and are relatively inexpensive.
Although electroluminescent lamps were known in the 1950s, these had a short lifetime and it was not until the 1980s that a flexible electroluminescent device was developed. However, this was used as an LCD backlight and only recently have practical electroluminescent displays become available.
Electroluminescent displays generally comprise a layer of phosphor material, such as a doped zinc sulphide powder, between two electrodes. It is usual for at least one electrode to be composed of a transparent material, such as indium tin oxide (ITO), provided on a transparent substrate, such as a polyester or polyethylene terephthalate (PET) film. The display may be formed by depositing electrode layers and phosphor layers onto the substrate, for example by screen printing, in which case opaque electrodes may be formed from conductive, for example silver-loaded, inks.
Examples of electroluminescent devices are described in WO 00/72638 and WO 99/55121.
An electroluminescent display of the general type described above is illuminated by applying an alternating voltage of an appropriate frequency between the electrodes of the lamp to excite the phosphor.
Commonly, the phosphors used in electroluminescent displays require a voltage of a few hundred volts.
Typically, such electroluminescent displays may have a capacitance in the range 100 pF to 50 nF.
Since only a small current is required, this comparatively high drive voltage can easily be produced from a low voltage DC supply by a circuit such as the well known “flyback converter”. This comprises an inductor and an oscillating switch arranged in series. In parallel with the oscillating switch, a diode and a capacitor are arranged in series. The switch oscillates between an open state and a closed state. In the closed state, a current flows from the DC supply through the inductor and the switch.
When the switch is opened, the current path is interrupted, but the magnetic field associated with the inductor forces the current to keep flowing. The inductor therefore forces the current to flow through the diode to charge the capacitor. The diode prevents the capacitor discharging while the switch is closed.
The capacitor can therefore be charged to a voltage that is higher than the DC supply voltage, and current at this voltage can be drawn from the capacitor.
In order to supply an alternating current to a load from a flyback converter, an H-bridge may be provided in parallel with the capacitor. In general, an H-bridge comprises two parallel limbs, each limb having a first switch in series with a second switch. On each limb between the first and second switches, there is a node, and the load is connected between the respective nodes of the limbs. Current can flow through the load in one direction via the first switch of one limb and the second switch of the other limb and in the other direction via the other two switches. The switches of the H-bridge are operated so that current flows through the load first in one direction and then in the other.
When discharging the capacitive load, it is known to provide a switched discharge path in parallel with the load to ground. This allows the charge stored in the capacitor to be dumped; the discharge path can be activated to provide a conductive path to ground and closed again when it is desired to stop discharging and recharge the load. In an alternative, it is known to discharge the load to form an auxiliary power supply for the switches of the control circuit (that is the H-bridge and the switch of the flyback converter) for the capacitive load; see for example United Kingdom patent application publication number GB2 405 270.
If the display is to be capable of selectively illuminating several different regions or “segments”, each segment will be provided with its own segment electrode at the rear of the display. All the segments typically share a common electrode in the transparent ITO electrode at the front of the display. Accordingly, it is known to drive each segment electrode with a “half-H-bridge” comprising a pair of switches in series, the electrode connected between the pair. One end of the pair is connected to the HV power supply, whereas the other end is connected to an appropriate reference voltage (typically ground). By closing one switch and opening the other, the segment electrode can be connected to either the HV power supply or ground.
Conversely, the common electrode is also connected to the power supply and ground by a half-H-bridge in the same manner. However, the half-H-bridge of the common electrode is generally operated in anti-phase with the segment electrodes of the segments that are to be lit, such that when the segments are connected to the power supply, the common electrode is connected to ground and vice versa. Accordingly, a HV AC signal can be generated across the segments of interest, thereby illuminating them.